In many sewage basin applications, pressure switches of conventional design are used to provide an on/off switching and an alarm signal when an alarm level is exceeded even while the pump is attempting to lower the level. Many of these designs utilize an “air bell” to isolate the switch port from the sewage environment. This “air bell,” which acts like an inverted glass under water, uses the compressibility of air in the bell to transmit level changes in the liquid at the opening of the bell. However, when small changes in liquid level, on the order of a few inches of water, need to be detected, there must be a vent to atmosphere to compensate for the atmospheric pressure changes, so that weather and elevation do not cause the set points to shift. This atmospheric venting offers an opportunity for moisture to get to the switch or its components.
A further problem with the air bell is that changes in the amount of gas in the air bell can also shift actuation points. One reason for air loss in the air bell is leaky fittings. Another reason is oxygen consumption due to decomposition of sewage materials in the basin, which can be especially troublesome when the sewage basin is only used for portions of the year, as with a summer cabin. A yet third reason could be the entry of methane or other decomposition gases into the air bell at the liquid-gas interface.
In the known prior art, these problems with air bells have been cured by lifting the level control system completely out of the sewage liquids and resetting it into place, recapturing the air in the bell. This solution has many problems, including the undesirability to have to open the sewage basin and to move the level control system.
It is therefore an object of the present invention to provide an environmentally sealed pressure level switch control in which at least the on/off switch is provided such that it does not use an air bell that must be vented to atmosphere.